1. Field of the Invention
The present invention relates to a composite dielectric, a composite dielectric sheet, a composite dielectric paste, a metal-layered composite dielectric sheet, a wiring board and a multilayer wiring board.
2. Related Background of the Invention
Recently, as information transmitted through electronic devices such as information and communication devices increases, wiring boards and the like mounted to these electronic devices have been required to be applicable to higher frequencies of GHz bands. As the frequency of transmitted signals increases, however, transmission loss tends to increase in the process of transmission, whereby electronic components may disadvantageously generate heat because of the thermal energy caused by the transmission loss, and so forth. In general, such a transmission loss is represented by the following expression:(transmission loss)=(coefficient)×(frequency)×(dielectric constant)1/2×(dielectric dissipation factor).For reducing the transmission loss, it is desirable that the dielectric constant and dielectric dissipation factor of the substrate material be lower.
As mobile electronic devices come into wider use, there is a strong demand for electronic devices to reduce their size and weight. For this purpose, it is preferable in a wiring board to be mounted that the wavelength of a signal to be transmitted be made shorter so as to reduce the wiring length, and the electrostatic capacity per unit area be increased so as to reduce the electrode area. Such wavelength and electrostatic capacity are represented by the following expressions:(wavelength)=(light velocity)/((frequency)×(dielectric constant)1/2);(electrostatic capacity value)=(dielectric constant in vacuum)×(relative dielectric constant of material)×(electrode area)/(insulating layer thickness).
Therefore, it is necessary for the substrate material to have a higher dielectric constant in order to shorten the wavelength and increase the electrostatic capacity value.
Hence, for satisfying demands for electronic devices to attain both higher frequency and smaller size, it is necessary to use a substrate material satisfying both characteristics of a higher dielectric constant and a lower dielectric dissipation factor.
Further, for reducing the size and weight of electronic devices, a high-density packaging technique is often used for connecting substrates, electronic components, and devices. Known as such a high-density packaging method is one applying cream solder to devices by using SMT (Surface Mount Technology), and then melting the solder by causing it to reflow, so as to bond the devices together. Recently, from the viewpoint of environmental problems and the like, lead-free solder has become mainstream. In case of applying the lead-free solder, a high temperature is necessary for the solder to reflow. Therefore, a material used for the substrate is desired to be resistant to a high temperature condition on the order of 260° to 280° C.
Characteristics required for the substrate material other than those mentioned above include thinning processability for electronic devices to reduce their size and weight, and lower water absorption for preventing the substrate from deteriorating with time by dielectric characteristics drift.
For responding to such demands, the following various substrate materials have been proposed. Namely, substrate materials such as polyimide, bismaleimide triazine, cyanate ester, polyvinyl benzyl ether, cross linking polyphenylene oxide, a mixed resin of an epoxy resin and an aromatic ester resin, a mixed resin of a phenol resin and an aromatic ester resin, an epoxy resin with a low water absorption and syndiotactic polystyrene, for example, are in use. However, these substrate materials do not satisfy all of the above-mentioned characteristics, and are often insufficient in terms of both characteristics of high heat resistance and low water absorption.
Attention has recently been focused on aromatic liquid crystal polyesters, which are thermoplastic crystalline engineering plastics, as a material adapted to realize characteristics of high heat resistance and low water absorption. Aromatic liquid crystal polyesters have such characteristics as excellent heat resistance and thinning processability, a low water absorption, and a low dielectric dissipation factor in a high frequency region. Accordingly, they are employed for electronic components such as connectors, and their application to a multilayer printed wiring board applicable to high frequencies is under consideration.
Such an aromatic liquid crystal polyester is processed into a sheet by injection molding or extrusion molding. However, in case of these molding methods, it is hard to yield a thin sheet, and the resulting sheet often exhibits anisotropy. Though a method which erases the anisotropy by applying a force in a direction perpendicular to the sheet extruding direction has been under consideration, sheets made of aromatic liquid crystal polyesters tend to exhibit a low dielectric constant in any case, and their dielectric characteristics are insufficient for use as a substrate material.
Therefore, as a substrate material which can achieve a high dielectric constant while taking advantage of such characteristics of aromatic liquid crystal polyesters, a dielectric paste containing an aromatic liquid crystal polyester, a predetermined solvent, and a dielectric powder has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2002-60595).